1. Field of the Invention
The present invention relates to a process and to a device for liquefying a compound A from a mixture of compound A and of one or more compounds B, each one of compounds B having a lower boiling point than compound A.
2. Description of the Prior Art
The prior art describes various processes for liquefying natural gas. In most of these processes, for example those described in patents U.S. Pat. Nos.4,490,867 and 4,445,916, liquefaction comprises a refrigeration stage and a gasoline extraction stage, followed by a liquefaction stage by refrigeration of the gasoline-freed gas using a mixture of coolants circulating in a closed loop. After the liquefaction stage, the unwanted non-burning compounds such as nitrogen and/or helium are extracted by expansion after the refrigeration stage. The flash gas from the low-temperature separator placed after expansion contains most of these non-burning compounds present in the feed. It can be used as a fuel gas because it generally contains a large fraction of combustible compounds initially present. The liquid coming from the low-temperature separator constitutes the commercial LNG. The liquefied natural gas is not recycled.
The expression "unwanted non-burning compounds" refers to the compounds which lower the calorific value of the gas and whose proportion is limited in the commercial natural gas.
According to another principle, some licensors use natural gas expansion, recompression and recycling stages which are generally coupled with preliminary external refrigeration stages by means of a mixture of coolants in a closed loop, as described in patent U.S. Pat. No. 5,363,655.
During these liquefaction processes, the natural gas, which is generally prerefrigerated by a first external cycle, is expanded, after gasoline extraction, through a series a turbines, recompressed and recycled. These processes do not allow use of natural gas with a notable nitrogen or helium content. In fact, above a given content, even low quantity, the nitrogen or the helium accumulate in the recycling loop and the process thus becomes non-economical or even technically impossible to implement.